Hi all,I'm done with modding a TWIN Famicom and I have an issue I can't get to troubleshoot.

I'm using the the original RF modulator port. The issue is I have small interferences particularly visible on the blacks. Like tiny white dots all over the place. Where it gets strange is that I can remove those interferences by plugging either of the original composite port (image or sound) to any composite port on my tv.

I've ruled out sync issue because the issue is exactly the same whatever sync I use. I have also rewired my mod with bigger wires. Made things better but not perfect.

I even isolated the video composite port by removing r214, but still, interferences are here and disappear when plugging a jack. To be specific the jack needs to be plugged both ways. If it's plugged to the Famicom but not to the TV the interferences are back.

I just don't understand how those ports I'm not even using could have that effect. Do you have any idea?

Exemples below:

Twin Famicom W/ NESRGB using RF Out Port. No RCA plugged in :

Twin Famicom W/ NESRGB using RF Out Port. Either RCA plugged in :

Last edited by romibraman on Fri Feb 16, 2018 2:51 am, edited 1 time in total.

1- That's interference at 10.7MHz. There's nothing in the Famicom that should emit energy at that band. (There is a crystal at 21.5MHz, but no-where is there anything that divides it by just two)

2- Composite over-the-air demodulation is supposed to be narrow-band enough that there couldn't be any energy at that frequency mixed in anyway

Maybe check the shield on the RF switch you're using. By plugging in either baseband composite or audio, you add a ground path, so my only guess directly from that symptom is that the coax is somehow damaged.

I'm not sure I follow what you mean by "There's two dots per NES pixel". The interference I'm talking about are those white dots all over the black area on the top picture.

Yeah, look at this bit:

Attachment:

4ztYwE6h_pixels_marked.png [ 34.13 KiB | Viewed 382 times ]

But you're talking about the random dots, not the grid. I just saw the grid, because I thought it more objectionable.

Anyway, if things get better when you connect composite video or audio, you've got problems with your cable having a lousy ground connection. You don't need to worry about the actual wires that are carrying the signal, but check the structurality of the shield and grounding pins on the connector and the cable itself.

Or possibly the quality of ground connection from the port to the famicom mainboard is lousy.

Anyway, if things get better when you connect composite video or audio, you've got problems with your cable having a lousy ground connection. You don't need to worry about the actual wires that are carrying the signal, but check the structurality of the shield and grounding pins on the connector and the cable itself.

A problem with the cable would make a lot of sense... It seems like an old one and it's the only Din8 cable I have.

The connector is brand new, (provided by Tim Worthington)

I have tried connecting the ground pin to different point on the NESRGB and the Famicom itself and it did make things better or worse.

Sorry for the dumb question but how do I test the "structurality of the shield".

Also, I'm still not sure I understand why connecting the composite would have an effect on another cable. Does it somehow establish the link between the ground of the TV and the ground of the Famicom (something the other cable doesnt do because it does not work correctly) ?

Again thanks a lot lidnariq.

PS : the double Pixel might be due to the fact that it was one of the latest Sony Trinitron out there so maybe it was some kind of feature.

When I did this the white dot were much more visible when the Din 8 ground was wired directly to the nesrgb ground than when it was wired to one of the composite port ground (on the second case the size of the wire is just a few centimeters as the composite port is just next to the Din 8).

In both cases connecting either of the composite port completely removed the artefact.

Certainly a "better" power supply (smoother) could reduce the amount of noise ... but the noise you see doesn't seem consistent with a lousy power supply. (Switching power supply or transformer-rectifier?)

I think I remember someone saying to be very careful about using a higher voltage PSU with the Twin Famicom because ... something something using the unregulated voltage and it being too high? But that's not a "noise" issue. If my memory is right, that's a "damaging something" issue.

I'd try adding extra ground connections between the A/V jack (both shield and the ground pin on the connector) and various places on the mainboard.

I almost messed up because of this because I thought pin 17 was in fact pin 19. If you look at my picture, you can clearly see the black wire going to pin 17 then next to it, pin 19 is not wired (as expected) and that's it. I have nothing else.

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